A growing need exists for large-scale energy storage systems that can support electrical transmission grids and enable the reliable implementation of intermittent renewable energy sources [1, 2]. Redox flow batteries are considered promising devices for these applications due to their scalability, durability, and transportability [3, 4]. Most flow batteries are based on aqueous electrochemistry and are thus limited by the electrochemical properties of water. Transitioning to organic electrolytes enables the development of flow batteries which can operate in larger windows of electrochemical stability, and can achieve higher energy densities and higher energy efficiencies.
Low-cost, scalable energy storage systems are needed to improve the energy efficiency of the electrical grid (e.g., load-leveling, frequency regulation) and to facilitate the large-scale penetration of renewable energy resources (e.g., wind, solar) [1,2]. While alternative energy technologies exist, they cannot be directly connected to the grid because of their variable output. Electrochemical energy storage may provide the best combination of efficiency, cost, and flexibility to enable these applications [5]. Of particular interest are redox flow batteries, which are rechargeable electrochemical energy storage devices that utilize the oxidation and reduction of two soluble electroactive species for charging (absorbing energy) and discharging (delivering energy) [6]. Unlike conventional secondary batteries, the energy-bearing species are not stored within an electrode structure but in separate liquid reservoirs and pumped to and from the power converting device when energy is being transferred. Because of this key difference, flow battery systems can be more durable than conventional battery systems as electrode reactions are not accompanied by morphological changes due to the insertion or removal of the active species and can be more scalable than conventional battery systems as the energy capacity may be easily and inexpensively modulated by varying the reservoir volume or the species concentration, without sacrificing power density. Thus, while flow batteries may not compete with compact lithium (Li)-ion batteries for portable applications (e.g., cell phones, laptops) due to lower overall energy densities, they are well-suited for large-scale stationary applications.
Since their inception in the 1960s, a large number of aqueous redox flow batteries have been developed including iron-chromium, bromine-polysulfide, vanadium-bromine, and all-vanadium systems [3, 4, 6]. Several aqueous hybrid systems also have been developed, where one or both electrode reactions are a deposition/dissolution process, such as zinc-bromine and soluble lead-acid systems. Though several of these aqueous technologies have been successfully demonstrated at the megawatt-scale, none have experienced widespread commercialization due to low energy densities, low round-trip energy efficiencies, and high costs. Indeed, all flow batteries based on aqueous electrochemical couples are limited by the electrochemical properties of water, which is only stable within a small potential window (typically 1.2-1.6 V) outside of which water electrolysis occurs. Employing non-aqueous electrolytes offers a wider window of electrochemical stability, which, in turn, enables flow batteries to operate at higher cell potentials (e.g., >2 V). If appropriate redox couples can be identified, operating at higher cell voltages leads to greater system energy (and power) densities and higher energy efficiencies. Moreover, as fewer cell units and ancillary parts would be required to achieve the same power output as an aqueous system, hardware costs would be significantly reduced and system reliability increased. In contrast to their aqueous counterparts, only a few non-aqueous flow batteries have been reported. The majority of the reported non-aqueous flow batteries are anion-exchange systems which employ single electrolytes composed of metal-centered coordination complexes [7-12]. Matsuda et al. demonstrated a system based on a ruthenium bipyridine complex with an open circuit potential (OCP) of 2.6 V [7]. Thompson and co-workers have investigated vanadium, chromium, and manganese acetylacetonate-based systems with OCPs of 2.2, 3.4, and 1.1 V, respectively [9-11]. Kim et al. recently reported systems with two different coordination complexes for the negative and positive electrode reactions based on nickel bipyridine and iron bipyridine as the negative and positive electrodes, respectively, with an OCP of 2.4 V [12]. Despite their promising cell potentials, these systems have been hampered by low efficiencies and the limited solubility of coordination complexes.
In general, Li-ion batteries have round-trip efficiencies of >95% and can have cell voltages over 4 V. Chiang and co-workers recently reported a high energy density semi-solid flow battery based on a slurry suspension of lithium intercalation materials [13]. Questions remain regarding long term durability, scalability, and cost of such systems, however.
High potential organic redox shuttles are employed in Li-ion battery packs to prevent overcharging of individual cells which can lead to thermal runaway and catastrophic failure [14]. Generally, the redox shuttle molecule activates at a defined potential slightly higher than the end-of-charge potential of the positive electrode. At this potential, the redox molecule oxidizes on the positive electrode, migrates to and reduces on the negative electrode, and then diffuses back to the positive electrode completing an internal ionic circuit, which holds the cell at a stable potential. First proposed in the 1980s, this technology has benefited from three decades of effort leading to a suite of robust materials [15-18]. During typical validation studies, overcharge materials are tested in Li-ion coin cells charged to twice the positive electrodes capacity (approximately 100% overcharge). During each overcharge cycle, individual molecules shuttle between the two electrodes hundreds of times and remain stable in their oxidized state, typically as a radical cation, for 1 to 10 seconds or more.
There is an ongoing need for new, more efficient, redox flow batteries. The present invention addresses this need by providing a non-aqueous redox flow battery based on oxidation and reduction of organic electroactive materials at the negative and positive electrodes and cation exchange involving transfer of cations such as alkali metal ions (e.g., lithium and sodium), and alkaline earth metal ions (e.g., magnesium and calcium) to balance charges resulting from the redox reactions.